This application claims the priority of German Application No. 198 55 250.5 filed Nov. 30, 1998, which is incorporated herein by reference.
The invention relates to a method for the image optimization of an X-ray image which is formed by X-rays after transilluminating an object. The X-rays exiting the object are detected and are processed in a computer as image data to form an image. The computer has an image memory. The image data are formed of image dots which are applied to the image memory.
Thus, objects are transilluminated by X-rays and the Xray image is made visible for an operator. As a rule, the transilluminated object (such as a piece of luggage) contains further items.
Upon transillumination of the items the X-rays are non-uniformly weakened, and the weakened radiation is made visible by suitable devices on monitors. The X-ray images are composed of image dots having various properties such as gray scale value and material value.
An image processing method for material recognition is described in German Application No. 198 12 055.9 filed Mar. 19, 1998. In that method signals detected during a partial illumination of a piece of luggage are subdivided as image data into image strips, are scanned and briefly stored. In such a procedure frequently dark regions appear in the image strips. Therefore, such dark regions of the X-ray image are visually evaluated in a known manner by an operator for which various optimizing functions are selected by manual actuation. Such a proceeding is time-consuming and is particularly undesired in case the transmillumination of hand luggage is performed, for example, at airports.
German Offenlegungsschrift (application published without examination) 43 30 787 describes a method for operating X-ray illuminating automats. The method disclosed therein provides for an automatic selection of measuring fields. A first image computer calculates the gray scale distribution of a test image to superpose the gray scale values on a principal image which is subsequently produced. The purpose is to obtain an optimal illumination within the gray scale ranges. Such a use of a test image is excessively time-consuming in case of transilluminating pieces of luggage.
German Offenlegungsschrift 44 09 790 discloses a method and an apparatus for a dynamic compression of multi-stage gray scale images. The input image values which lie under a selectable, approximately average signal threshold are inverted. The image values which are recognized as absolute values above such a signal threshold are retained. The result of such a conversion of the input image values is an intarsia-like positive/negative image with a closed line clearly characterizing the monotony jump of the gray scale value representation and with an extra halved dynamic circumference. This process is used with preference in mammography. Such a process, however, cannot be used for evaluating a plurality of transilluminated material in a piece of luggage.
It is an object of the invention to provide an improved method for the image optimization of X-ray images which automatically and optimally reproduces regions which are difficult to identify and thus minimizes the time needed for such an identification.
This object and others to become apparent as the specification progresses, are accomplished by the invention, according to which, briefly stated, the method of optimizing an X-ray image representing an article transilluminated by X-rays, includes the following steps: detecting the X-rays leaving the article after transillumination thereof by the X-rays; processing, in a computer, the detected X-rays as image dots to obtain an image; applying the image dots to an image memory; scanning the image dots according to gray scale values thereof; comparing the scanned gray scale values with a desired threshold gray scale value; upon finding an image dot which is other than the desired threshold gray scale value, counting subsequent image dots to obtain an image zone A; determining the gray scale values of the image dots of the image zone A; and expanding the gray scale values of the image dots of the image zone A to higher image dot values for effecting a local brightening of the X-ray image in the image zone A by the higher image dot values.
The invention is based on the principle to automatically determine regions of the X-ray image that are difficult to identify, for example, dark regions, and to locally brighten such regions. The dark regions are reproduced brightened on a monitor without brightening the other lighter ranges at the same time. The local brightening of the dark regions is effected by expanding the gray scale values of the image dots which form the X-ray image. By brightening the dark ranges hidden objects may be detected.
The optimized process is involved in the image evaluation only at a time when a detected gray scale value falls below a threshold value or is equal to or greater than such threshold value and if a certain number of image points are determined which directly adjoin one another. It is, however, taken into consideration that, for example, excessively small zones should not cause a brightening or image optimization because in this manner the obtained X-ray image would be too crowded.
It is a further possibility to utilize, for the determination of the number of image points, not only directly adjoining image points which fall below a predetermined gray scale threshold, but also image points which fall below a certain gray scale threshold and are not immediately adjoining; that is, they may be separated by two or three image points which lie above the gray scale threshold. In this manner blurred zones too may be detected in case they contain a number of bright image dots.
It is a further possibility to examine only every third or fourth image dot as to whether or not its gray scale value exceeds the threshold. If the gray scale value of the third image dot falls below the threshold and thus is of identical magnitude as the previously evaluated image dot gray scale value, a summation of the omitted image dots 1, 2 (or 1, 2 and 3) follows for reproducing a complete image. If, however, greater differences are present, then the last three or four image dots are scanned and evaluated anew. In this manner the processing speed in the system is increased.
Since apart from the absorption values, material values may also be determined by the detectors, it is feasible to provide for a material-dependent image optimization. In such a method the image dots are counted, scanned concerning a specific property and examined concerning their gray scale value, and the scanned zone is brightened.
FIG. 1 is a schematic side elevational view with block diagram, illustrating a measuring system.
FIG. 2 is a block diagram illustrating a system for performing the optimizing process according to the invention.
FIG. 3a illustrates a reproduction of an image on a monitor without image optimization.
FIG. 3b shows a reproduction of an image on a monitor with image optimization.
FIG. 4 is a schematic side elevational view with block diagram, illustrating a measuring system different from that shown in FIG. 1.